Pleiotropy occurs when a single gene influences multiple phenotypictraits. Consequently, a new mutation in the gene will have an effect on all traits simultaneously. This can become a problem when selection on one trait favours one specific mutant, while the selection at the other trait favours another mutant.

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Pleiotropy describes the genetic effect of a single gene on multiple phenotypic traits. The underlying mechanism is that the gene codes for a product that is for example used by various cells, or has a signalling function on various targets.

The easiest way to explain the mechanism is to use an example. A classic example of pleiotropy is the human disease PKU (phenylketonuria). This disease causes mental retardation, and reduced hair and skinpigmentation. The cause is a mutation in a single gene that codes for an enzyme (phenylalanine hydroxylase) that converts the amino acidphenylalanine to tyrosine, another amino acid. The mutation results in a no or reduced conversion of phenylalanine to tyrosine, and phenylalanine concentrations increase to toxic levels, causing damage at several locations in the body.

Antagonistic pleiotropy refers to a situation in which a single gene creates multiple competing effects, such that beneficial effects of a trait created by the gene are offset by 'losses' in other traits. One example is a theory of aging first developed by G. C. Williams in 1957. Williams suggested that one gene is responsible for increased fitness when young at the expense of fitness later in life (i.e. aging). Another example might be a gene in a bacterium which confers increased glucose utilization efficiency at the expense of other carbon sources (such as lactose).